DHAKA (Reuters) - Bangladesh police are trying to determine the whereabouts of at least 260 young men who have been missing for a year or more, a security officer said on Wednesday, in an effort to track militants after a deadly attack this month.

Five young men killed 22 people, most of them foreigners, in an attack on an upmarket Dhaka cafe on July 1 claimed by Islamic State. Three of the attackers were from affluent Dhaka homes who had broken off contact with their families months ago.

Authorities have blamed the attack on a domestic militant group, but security experts say the scale and sophistication of the assault suggested links to a trans-national network.

After the attack, the government appealed to families to contact authorities if their sons had disappeared.

A senior officer with the police-led Rapid Action Battalion (RAB), which is involved in the counter-terrorism effort, said a list of 260 missing young men had been compiled from reports from families and intelligence tip-offs.

"Some of the missing youngsters are sons of retired or serving army officials, bureaucrats and businessmen," Mufti Mahmud Khan, chief of the RAB's legal and media wing, told Reuters.

Information was still being gathered and the number of missing was expected to grow, he added.

The RAB list posted on its Facebook page includes a 16-year-old, a doctor whose father was in the army and a man arrested in 2013 with explosives who went to Malaysia while on bail.

Mostly Muslim Bangladesh has faced a series of attacks on liberal bloggers, university teachers and members of religious minorities over the past year.

Lawyers said a court on Wednesday charged five suspected members of a banned militant group, the al Qaeda-inspired Ansarullah Bangla Team, over the March 2015 murder of an online critic of religious extremism.

Bystanders in Dhaka, the capital, seized two of the attackers as they tried to flee after hacking to death the blogger, Washikur Rahman, 27, and police later arrested a third suspect.

The three men now in jail pleaded not guilty, public prosecutor Salauddin Hawlader told reporters, while the others, who are on the run, are to be tried in their absence.

Islamic State has warned that the violence would continue until Islamic law was established worldwide, saying in a video the Dhaka cafe attack was just a hint of what was to come.

Three young Bangladeshi men who appeared in that video posing as fighters also appeared on the list of missing.